An indexable inserts is mounted on a cutting tool holder for machining a workpiece and may take various shapes such as square and triangle with an equal corner angle all around the insert, or rhombic or rhomboidic shapes with acute nose point angles.
In general, a machining tool mounted with this indexable insert is used for drilling, milling, and turning. In these various machining methods, the cutting insert is secured onto the seat of the tool holder by a clamping means and provides a cutting edge to contact the workpiece.
A clamping means to secure an insert with a central hole to a tool holder has been disclosed in EP 0759826 B1. This clamping means intends to keep the insert in place in the pocket so that the clamping force is acting on both the top surface and in the central hole wall of the insert without any tendency of coming apart from its underneath supporting surface of the seat of the tool holder. And the rear end portion of this clamp is adapted to come into abutment with an inclined rear surface provided on the holder body.
However, this clamp is characterized by a point contact between the hole wall of the insert and the front clamp projection at a point distantly provided from the top surface of the insert by means of arranging a forward protrusion having an axis of symmetry that is oriented at an inclined angle(β) relative to the axis of the hole. In this clamping structure, the contact point pressing the hole wall has only an infinitesimal area. Accordingly, when the cutting tool is used for a prolonged period of time, the contact point is susceptible to wearing and the clamp is got loosened. Hence, there has been a concern that the clamping becomes too loose for an appropriate cutting operation if the clamp is not tightened further once in a while.
Also, with the above insert, the flat underneath surface is not guaranteed to achieve a flat surface abutment against the top surface of the insert. In other words, as the contact position of the end surface of the rear protrusion on the inclined contact surface is varied depending on the manufacturing variations of inserts or clamps, the contact between the flat underneath surface and the top surface of the insert may not be an actual planar contact, but may result in a point contact at a forward border position “A” or a rearward border position “B” of the flat surface as illustrated in FIG. 6. If this is the case, the contact point is also susceptible for wearing and the clamping gets loosened under a prolonged use of the tool. Especially, when the contact is made at a forward border position “A”, a force lifting the rear portion of the insert apart from the bottom face of the insert pocket due to a shear force applied on the insert nose is not effectively counteracted, and an ultimate insert failure may result. Also, since shear force is mainly applied on one active edge of the two edges adjacent to the nose, there is a tendency of lifting a portion of the insert opposite to the active edge apart from the bottom face of the insert pocket. However, the aforesaid clamp is inefficient for suppressing this tendency, and this becomes more problematical if the insert is in triangular shape where there is only one upstanding support surface of the insert pocket.
On the other hand, in Korean registered patent no. 0450439, the inventor proposed a contact structure, in which a contact portion formed on the inner side of the forward protrusion has a surface contact over a certain length with the inner wall of the central hole of an insert so that wearing is reduced. Also, pressing portions of flat surface are provided on both flanks of the forward protrusion to press down the insert.
However, in this structure, as the end surface of the rear protrusion is displaced on the inclined contact surface, a smooth surface contact is not achieved between the contact portion and the inner wall of the central hole but a point contact may occur at a top position “C” or a bottom position “D” as illustrated in FIG. 7. In this case, the contact point is also susceptible for wearing and the clamping gets loosened under a prolonged use of the tool. Especially, when the contact is made at a top position “D” the contact point is so close to the upper surface of the insert that the support point for transverse clamping force becomes unstable. Also, since pressing points are made around flank rim of the central hole, the force to counteract a force lifting the rear portion of the insert apart from the bottom face of the insert pocket due to a shear force applied on the insert nose is not sufficient and insert failure may occur ultimately.